The economic relationship between human and ocean is developing further and further in the order of “point-line-plane-body”, and such development is becoming faster and faster. The ocean is becoming more and more important and has an increasingly large effect on human, ranging from human survival level to economy level, and even rising to the level of politics and international relationship. However, the ocean monitoring capability of human always lags behind their oceanic practical activities, and now basically stays on the level of “point” and “line”, which cannot meet the requirement of monitoring the “plane” and “body” of the ocean in real-time.
The poor ocean monitoring capability is a major problem having affected the development of our country's oceanic economy, the prevention of oceanic disasters, and national safety and unity for a long time. As a non-contact shore base remote sensing device with the advantages of large-area, all-weather and low-cost, high frequency ground-wave radar is the best tool for realizing the real-time ocean monitoring on the “plane” level. However, if the effective supervision of the main coastal sea area throughout the country is to be realized, it will be necessary to arrange tens of or even hundreds of ground-wave radars of different type at a coastal area so as to form a radar network and perform detection. This has not only solved the capability insufficiency of our country's traditional ocean monitoring instrument, greatly improved the capability of conducting real-time monitoring for the ocean environment around our country, and realized a new “generation” of our country's ocean monitoring capability; meanwhile a large amount of expenditure for infrastructure construction can be saved.
The detection range of existing shore base medium and long range ground-wave radar device is usually 200 km and the detection range of short range ground-wave radar device is smaller; maritime international law specifies 200 nautical miles as exclusive economic zone, so obviously it is difficult to meet the requirement of exclusive economic zone monitoring by utilizing the ground-wave radar device of shore base, and the national strategy of collecting pelagic information and data is made clear in our country's Deep and Open Sea Development Plan. Therefore, it is necessary to realize the monitoring for high sea, and conducting real-time monitoring for high sea beyond 200 km by researching and developing a buoy-type high frequency ground-wave radar system etc. is the best supplement to the existing shore base ground-wave radar. Building an unmanned ground-wave radar system platform with fully-automatic working capability is particularly important for monitoring the ocean environment, which is a passage of strategic importance.
Presently, the insufficiency of high frequency ground-wave radar is obvious in terms of detection range, detecting element precision and applicability etc., mainly including the following five aspects:
A. The effective coverage of radar is insufficient. Coastal economic activities as well as disaster prevention and reduction require a good knowledge of the state of the ocean within hundreds of kilometers or even thousands of kilometers, but the relatively effective detection range of currently-deployed ground-wave radar is usually within 150 km. The increase in detection range is usually realized by reducing working frequency and increasing emitting power. The reduction of working frequency means the increase of radar antenna dimension and device complexity as well as the reduction of result resolution and result precision, thus the capability of detecting medium and short dimension gravity wave information will be weakened. The increase of emitting power reduces the reliability of device and the applicability of electromagnetic environment, and importantly, the speed of the increase of detection range detection range will rapidly reduce as the emitting power increases; if the power is improved from 10 kW to 100 kW, the detection range will only increase by 30˜40 km, and the operation cost of radar will increase exponentially as the detection range becomes long. Therefore, the detection range cannot be greatly improved just by the ground-wave radar itself. As an effective approach to increasing the detection range of ocean monitoring radar, buoy-type high frequency ground-wave radar can greatly improve the detection range of high frequency radar system by means of the long-distance transmitting mechanism of sky-wave, the networking of distributed radar, and the integration of sky-wave and ground-wave.
B. The amount of detected information is insufficient, so the accuracy and reliability of the detection result is not high. Up to now, all the domestic ground-wave radars for ocean detection belong to single station radar with a common station for receiving and sending, and the radar only extracts information from the backward scattered echo wave of sea surface, and the information contained in the backward scattered echo wave, especially the direction information on physical parameters (e.g., flow direction, wind direction and wave direction etc.) is extremely limited, leading to large direction error of detected parameters and the serious reduction of radar detection accuracy, thus the result is not usable. This problem is obvious in comparison tests and practical use of high frequency ground-wave radar in recent years, and is a fundamental technical bottleneck which restricts ground-wave radar from being used more widely. Thus, it is urgent to develop a radar system capable of obtaining the non-backward scattered information of sea surface, i.e., a distributed high frequency radar system in the mode of “multiple sending and multiple receiving” composed of a plurality of participating radars at different. It is not a simple combination of the inversion result from multiple single-station radars, but obtains relatively comprehensive information at the raw signal level. It can significantly improve the detection accuracy of wind, wave and current to ensure our country's ocean detection with high frequency ground-wave radar to enter the phase of operational application. Meanwhile, buoy-type high frequency ground-wave radar can also provide a large amount of detection information which is based on open-sea detection and makes up for the shortage of the high sea data amount of shore base radar.
C. The anti-interference ability of radar system is insufficient. The high frequency radar works in a frequency range with strong interference. In particular, ionosphere interference, radio frequency interference and transient impact interference are serious, among which the ionosphere interference is the most serious problem faced by the ground-wave radar of middle and low latitude area, and is one of the major difficulties restricting the application of ground-wave radar. Presently, there is no effective way to restrain ionosphere interference in the world. The ionosphere has high interference intensity and very complex signal characteristics. A great deal of research domestically and abroad indicate that ionosphere interference cannot be controlled easily in single station radars, and a distributed high frequency radar system with “multiple sending and multiple receiving” as well as the detection information of ionosphere vertical measurement instrument and oblique measurement instrument have almost become the only choice for improving the ionosphere interference resistant capability of an over-the-horizon radar. One advantage of long-distance detection via sky-wave transmitting and ground-wave receiving is that the interference of the F region of the ionosphere will be avoided, and the usual interference of the F region of the ionosphere in the echo wave spectrum of ground-wave radar will not exist in the received echo wave spectrum. Certainly, the electron density change or movement of ionosphere reflection points during sky-wave transmission will cause the shifting and widening of the spectrum, but such influence can be compensated via the sea surface information obtained by ground-wave.
D. Conventional high-frequency ground-wave radar system conducts detection in a single mode, therefore a precise and detailed detection result cannot be obtained. At present, most high-frequency ground-wave radars for ocean detection are arranged near coastline, thus the choice of location is usually limited by location-specific factors, and only the backward scattered echo wave of its own can be received, the detection is done in a single mode, and therefore the precision of result is not high. The distributed high frequency radar system can provide a variety of detection modes, which helps to realize the detailed observation of near-shore.
E. At present, the high-frequency radio band is very crowded, and frequency resource is very precious. Medium and long range radars usually work in the frequency range of 5˜15 MHz, which range has been crowded with a large number of civil communication channels. When conventional single-station radar is used for detection, every radar station of the same sea area needs to work on different working frequency to avoid mutual interference. Therefore, the current frequency resource of high frequency range can no longer support the addition of more single-station radar for ocean monitoring, which has become one of the most important social factors restricting the wide use of ground-wave radar. By contrast, distributed high frequency radar networking can solve this problem, and all the radars of the same sea area use the same frequency, which greatly saves the occupied frequency band of the radar network, reduces the possibility of being interfered, and facilitates the popularization of over-the-horizon radar in ocean environment monitoring.
The buoy-type high frequency ground-wave radar can overcome the foregoing drawbacks, break the limit that conventional ground-wave radars can only be arranged along coastline, extend the detection range to high sea area by receiving sky-ground mixed path echo wave, and greatly enhance the flexibility of radar system deployment. Meanwhile, this system has expandability and the potential for accommodating further radar nodes, making diversified application possible.